The duct furnace with ribbon burners and an oval tubular heat exchanger is a low cost warm air heating device used in commercial and industrial heating. Applications include unit heaters, ducted warm air heating systems and ventilation make-up air heaters. In certain applications, particularly ventilation make-up air heaters, it is desirable to be able to modulate the heating output of the duct furnace by varying the firing rate of the burners. One purpose of modulating the output of a ventilation make-up air heater is to provide constant make-up air delivery temperature over the normal range of outdoor ambient temperatures. To best meet this objective, it is desirable to be able to modulate the burners over as wide a range as possible.
In a conventional indirect fired make-up air heater, an induced draft blower is used to provide essentially constant combustion air flow in a variety of configurations ranging from sealed combustion to roof top mounted. In the latter case, the induced draft system minimizes the effect of wind speed and direction on combustion air flows. In order to provide a more constant heated make-up air delivery temperature, stepped and continuous modulation is available in this type of unit, but generally is limited to turn-down ratios of 2:1, i.e., the minimum firing rate is 50% of the maximum firing rate. At firing rates below this level, both the combustion quality and the thermal efficiency deteriorate below levels that are acceptable with respect to industry safety certification standards. In particular, carbon monoxide (CO) levels increase.
As the firing rate of a partial pre-mix burner, such as a ribbon burner, is reduced without reducing the combustion air flow rate, a point is reached where the cool secondary air flow quenches the combustion of the outer portions of the flame, causing the aforementioned increase in CO levels. If the combustion air flow rate is reduced in tandem with the firing rate, acceptably clean combustion can be maintained to a lower firing level, before other quenching effects, such as the cooling effect of burner walls and heat exchanger walls cause CO levels to rise. In a heating device certified for sale in the U.S., reduction of the combustion air flow rate can be constrained by the requirement to sense an obstruction to combustion air flow, either a blocked flue vent or a blocked air intake.